“In the zone” means being completely absorbed in English, or reaching the peak of concentration.
It is often used when sports players are fully focused on a game and performing at their best, or when a researcher is so focused on preparing a presentation that they lose track of time.
It goes beyond simply being concentrated and is used to describe a psychological flow state in which the best performance can be achieved.

3 everyday conversation examples
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I was studying for the exam and got completely in the zone. → I was studying for the exam and got completely in the zone.
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He was in the zone, scoring points one after another in the game. → He was in the zone, scoring points one after another in the game.
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I was writing and didn’t notice the time because I was in the zone. → I was writing and didn’t notice the time because I was in the zone.
3 academic/professional English examples
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During the presentation, I forgot my nerves and stayed in the zone. → During the presentation, I forgot my nerves and stayed in the zone.
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In the research discussion, he was in the zone, answering sharp questions consecutively. → In the research discussion, he was in the zone, answering sharp questions consecutively.
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During the surgical training, I was in the zone and missed no details. → During the surgical training, I was in the zone and missed no details.